<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>用户注册</title>
</head>
<body>
<form method="post">
    <label>用户名：</label><label><input type="text" name="username"></label><br>
    <label>密码：</label><label><input type="password" name="password"></label><br>
    <label>确认密码：</label><label><input type="password" name="retryPassword"></label><br>
    <input type="submit" value="提交">

    // get_flashed_messages是函数，要加上小括号执行
    {% for message in get_flashed_messages() %}
        {{ message }}
    {% endfor %}
</form>
</body>
</html>